I could totally see how somebody can say that it is not a disease but sheer laziness, however, what brings on the laziness?

A thyroid condition or other physiological causes can impact the ability to "just get off your ass." Furthermore so can a mental condition such as depression. Not to mention the impact of medication on a person's metabolism or even genetic makeup.

Seems it is rather easy for someone to sit back and call people lazy when they are not predisposed to conditions that could impact metabolism etc.

Who's the lazy one?

The one that eats whatever, does no exercise, and still gains no weight or the one that follows a diet, exercises, and because of a medical condition gains weight.